1. Field of the Invention
The present invention relates to a method of manufacturing a substrate, particularly to a cavity-down plastic ball grid array (CD-PBGA) substrate.
2. Description of the Prior Art
With the sophistication of IC devices, the complexity of semiconductor dies has increased, leading to a greater number of slots for electric signals, plug-switches and conductive lines. This has lead to a variety of high-density substrates for packaging semiconductor dies, including CD-PBGA substrates. CD-PBGA substrates are used for ball grid array packaging, which has a excellent thermal dissipation characteristics. The traditional CD-PBGA substrate has between 300 to 800 ball counts, with an operating power of 5 to 10 watts or higher. The thermal dissipation capabilities of a CD-PBGA substrate can be improved using forced-air convection, such as by adding a cooling fan to the top of the CD-PBGA substrate with attached heat slug.
There are many prior art inventions in the field of cavity-down ball grid arrays (CD-BGA). For example, U.S. Pat. No. 5,027,191 provides a cavity-down padding array substrate for packaging a semiconductor die, and U.S. Pat. No. 5,420,460 provides a thin CD-BGA packaging structure for wire bonding.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a diagram of a traditional CD-PBGA substrate 10, and FIG. 2 is a cross-sectional view of the CD-PBGA substrate 10 of FIG. 1. The CD-PBGA substrate 10 made according to the prior art comprises an organic substrate 12 and a Cu heat spreader 14. The organic substrate 12 has in its center a squared or rectangular cavity for holding an IC die 18. When the CD-PBGA 10 is positioned on a printed circuit board (PCB), the Cu heat spreader 14 will be positioned on top away from the printed circuit board so as to facilitate heat radiation.
As shown in FIG. 1 and FIG. 2, one side of the organic substrate 12 comprises a plurality of bonding fingers 20, solder ball pads 22, and conductive interconnects 23 (FIG. 1 only shows a portion of the conductive interconnects 23) for electrically connecting the bonding fingers 20 to the solder ball pads 22. The organic substrate 12 has a Cu layer (Cu interconnects layer) 24 on its opposite side, and it has a plurality of conductive vias 26 for electrically connecting the bonding fingers 20, the ball pads 22, and the conductive interconnects 23 to the Cu layer 24. A solder mask 28 covers the organic substrate 12 to protect the packaging substrate 10, to isolate the conductive pads 20 and 22, and to insulate the conductive interconnects 23. The surface of each conductive pad 20, 22 is plated with a layer of nickel 30 and a layer of gold 32. An adhesive layer 34 is used to bind the organic substrate 12 to the Cu heat spreader 14. The adhesive layer 34 is usually an epoxy-based prepreg. Additionally, the surface of the Cu heat spreader has a Ni-plated finish 36 for protecting the Cu heat spreader 14 and to prevent oxidation.
After the IC die 18 is fitted, with the help of epoxy, into the cavity 16 of the substrate 10, the IC die 18 is, by wire bonding, electrically connected to the bonding fingers 20 through a plurality of conductive interconnects 38. The cavity 16 is then filled to seal in the IC die 18. A Pb/Sn or Sn solder ball 40 is fixed onto each of the solder ball pads 22, in order to bond the substrate 10 to the printed circuit board (not shown). Signals from the IC die 18 are transmitted through the conductive interconnects 38 to the bonding fingers 20 of the substrate 10, and through the conductive interconnects 23 to the solder ball pads 22 (or following a route from the conductive interconnects 23 to the conductive vias 26, the Cu interconnects layer 24, the conductive vias 26, and to the conductive interconnects 23). Finally, current from the IC die 18 is transmitted to the printed circuit board through the solder balls 40. Following the same route in reverse, signals are transmitted back to the IC die 18 from the printed circuit board.
The method of manufacturing the substrate 10 according to the prior art is to first separately make the organic substrate 12 and the Cu heat spreader 14, and then to combine the two. According to the conventional method, the bonding fingers 20, the solder ball pads 22, the conductive interconnects 23, Cu conductive interconnects layer 24 and the conductive vias 26 are made on the organic substrate 12 without the cavity 16. The solder mask 28 is then coated onto the substrate 12 on the same side as the ball pads 22 and bonding fingers 20. An etching process is performed to transfer an appropriate pattern to the solder mask 28.
After the solder mask 28 is made, a tape or film (not shown) is adhered to the other side of the organic substrate 12 (the same side as the Cu interconnects layer 24) before performing a single-sided Ni/Au plating process (on the same side as the conductive interconnects 23). The plating process is performed by plating each of the conductive pads 20 and 22 of the organic substrate 12 with a Ni layer 30 that is approximately 5 microns thick, and over which is plated a gold layer 32 that is approximately 0.5 microns thick. Upon the completion of the plating process, the tape or film is removed, and a squared or rectangular cavity 16 is cut into-the center of the organic substrate 12, followed by a cleaning process.
After cutting out the cavity 16, the manufacturer performs a single-sided Cu surface pre-treatment. An oxide layer 42 is formed over the Cu layer 24 of the organic substrate 12 in order to increase the surface adhesion of the organic substrate 12 by utilizing the coarse nature of the oxide layer 42. The oxide layer 42 can comprise black oxide or brown oxide.
When making the organic substrate 12, the manufacturer can at the same time fix a tape or a dry film (not shown) onto one side of the Cu heat spreader 14 before performing a single-side Ni plating process on the opposite side. After the Ni-plated finish 36 is formed, the tape or dry film is removed from the Cu heat spreader 14, and another single-side Cu surface pretreatment process is performed, in which an oxide layer 44 is formed on one side (the same side as the Ni-plated finish 36) of the Cu heat spreader 14 to increase the surface adhesion of the Cu heat spreader. The oxide layer 44 may comprise black oxide or brown oxide.
Please refer to FIG. 3 and FIG. 4. FIG. 3 shows the thermal laminating process in making the substrate 10 according to the prior art, and FIG. 4 is a view of removing a release film 48 and a filler film 46 after completing the thermal laminating process. After the organic substrate 12 and the Cu heat spreader 14 are made as described above, a thermal laminating process is performed to laminate the two pieces.
As shown in FIG. 3, a filler film 46 is fixed onto the organic substrate 12 to prevent the sticky adhesive layer 34 from flowing into the cavity 16 during the thermal laminating process. The filler film 46 may comprise polyethylene or silicone rubber. After the completion of the thermal laminating process, the filler film 46 must be removed completely, so before placing the filler film 46, the manufacturer places a release film 48 over the organic substrate 12. The release film 48 can be peeled from the organic substrate afterwards so as to help with the removal of the filler film 46.
Before performing the thermal laminating process, in addition to placing the release film 48 and the filler film 46, the manufacturer must also cut a cavity 50 in the center of the sticky adhesive layer 34 that corresponds to the cavity 16 to prevent the sticky adhesive layer 34 from remaining in the cavity 16 when the substrate 10 is completed.
After the preparations described above are completed, the filler film 46, the organic substrate 12 with the cavity 16, the adhesive layer 34 with the cavity 50 and the Cu heat spreader 14 are laid over one another and undergo a thermal laminating process to form the substrate 10. As shown in FIG. 4, the releasing film 48 is peeled from the substrate 10, which also removes the filler film 46. A thermal treatment process is then performed on the substrate 10 so as to control the warpage of the substrate 10 and to further cure the adhesive layer 34, tightly binding the organic substrate 12 and the Cu heat spreader 14 together.
So far, the substrate 10 is made by manufacturing a plurality of CD-PBGA substrates on a single substrate sheet (not shown). The substrate sheet is then cut into many individual substrates 10. The manufacturer can either form a plurality of tooling holes before or after laminating the substrate sheet, depending on the manufacturing needs. After cutting, the manufacturer inspects each substrate 10 to ensure that each substrate 10 conforms to industrial standards.
There are, however, several problems in this prior art method of making the CD-PBGA substrate 10:
1. Before plating Ni on one side of the Cu heat spreader 14, the manufacturer must fix a tape or film on its opposite side and then remove it after the plating process is done. Both the fixing and removing processes complicate the Ni plating process and result in high production cost.
2. The Ni plated layer 36 on the Cu heat spreader 14 is a source of high stress to the Cu heat spreader 14, affecting the warpage of the substrate 10, and may result in unevenness in the substrate 10.
3. The coefficient of thermal expansion (CTE) of a typical epoxy compound that is used to bind the IC die 18 is between 50 to 60 ppm/xc2x0 C. The CTE of the Cu heat spreader is about 17 ppm/xc2x0 C. These two values are not match to each other. In subsequent packaging processes, and in the process of soldering the substrate 10 to a printed circuit board, the substrate suffers high temperatures. The temperature of the IC die 18 rises during these processes. The high temperatures affect the epoxy compound and the Cu heat spreader 14 differently, resulting in a higher stress in the epoxy compound (underneath the IC die 18). The epoxy compound is thus not being able to effectively reduce the stress between the IC die 18 and the Cu heat spreader 14.
4. In manufacturing the substrate 10 according to the prior art, the Ni-plated finish 30 and the gold-plated finish 32 on the bonding fingers 20 and on the solder ball pads 22 are formed prior to the thermal lamination process. This increases the possibility of contamination or deterioration of the surface of the gold-plated finish 32.
5. According to the prior art, the adhesive layer 34 with its cavity 50 is used to bind the organic substrate 12 and the Cu heat spreader 14, but the process of making a precise cavity 50 is difficult and expensive. Additionally, to precisely align the organic substrate 12 having cavity, the adhesive layer 34 having cavity and the Cu heat spreader 14 is also a difficult task. Also, the application of the releasing film 48 and the filler film 46 in the thermal laminating process is inconvenient and uneconomical.
It is therefore a primary objective of the present invention to provide a method of manufacturing a CD-PBGA substrate to correct the weaknesses described above and to reduce production costs.
In a preferred embodiment, the present invention comprises an organic substrate and a heat spreader. The method involves first making an organic substrate and a heat spreader separately, and to then combine the two parts with a partially cured liquid-type adhesive layer. In making the organic substrate, a solder mask layer is first formed on one side of an organic substrate, then a cavity for holding an IC die is cut out. A surface pre-treatment process is then performed to the copper layer on the opposite side. A black ink layer is layered on one side of the heat spreader, and a second black ink layer is formed within a predetermined area on the opposite side. The predetermined area is reserved for positioning the IC die, and has a plurality of heat dissipating pads. Next, a liquid adhesive printing or coating process and a partial curing process are performed, which forms a solidified liquid-type adhesive layer outside the predetermined area of the Cu heat spreader. The organic substrate is then laminated to the Cu heat spreader at a high temperature. Finally, a Ni/Au layer is simultaneously plated onto a plurality of conductive pads and heat dissipating pads, fingers of the substrate.
It is an advantage of the present invention that performing the thermal lamination process before the Ni/Au plating process reduces the possibility of damage and contamination to the gold plating of the Ni/Au finish. The black ink layer is formed on the Cu heat spreader by way of a stencil printing process or other coating process, so that the process does not require any tapes or films to cover one side of the Cu heat spreader (for the purpose of single-side Ni plating), thus reducing production costs. Also, the use of a stencil-printed liquid-type adhesive layer to bind the organic substrate to the Cu heat spreader further simplifies the manufacturing process and reduces the materials used.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.